Role of Mitochondrial and Cytosolic Folylpolyglutamate Synthetase in One-Carbon Metabolism and Antitumor Efficacy of Mitochondrial-Targeted Antifolates

Folate-dependent one-carbon (C1) metabolism encompasses distinct cytosolic and mitochondrial pathways connected by an interchange between serine, glycine and formate. In both the cytosol and mitochondria, folates exist as polyglutamates with polyglutamylation catalyzed by folylpolyglutamate synthetase (FPGS), including cytosolic and mitochondrial isoforms. Serine is metabolized by serine hydroxymethyltransferase (SHMT) 2 in the mitochondria and generates glycine and C1 units for cellular biosynthesis in the cytosol. AGF347 is a novel pyrrolo[3,2-d]pyrimidine antifolate that targets SHMT2 in the mitochondria, and SHMT1 and de novo purine biosynthesis in the cytosol. FPGS is expressed in primary pancreatic cancer specimens and FPGS levels correlate with in vitro efficacies of AGF347 toward human pancreatic cancer cells. MIA PaCa-2 pancreatic cancer cells with CRISPR knockout of FPGS were engineered to express doxycycline-inducible FPGS exclusively in the cytosol (cFPGS) or in both the cytosol and mitochondria (mFPGS). Folate and AGF347 accumulations increased in both the cytosol and mitochondria with increased mFPGS but were restricted to the cytosol with cFPGS. AGF347-Glu₅ inhibited SHMT2 ~19-fold greater than AGF347. By metabolomics analysis, mFPGS stimulated the C1 flux from serine in the mitochondria and de novo purine and dTTP synthesis far greater than cFPGS. mFPGS enhanced in vitro inhibition of MIA PaCa-2 cell proliferation by AGF347 (~30-fold) more than cFPGS (~4.9-fold). Similar results were seen with other pyrrolo[3,2-d]pyrimidine antifolates (AGF291, AGF320); however, elevated mFPGS adversely impacted inhibition by the non-classical SHMT2/SHMT1 inhibitor, SHIN1. These results suggest a critical role of mFPGS levels in determining anti-tumor efficacies of mitochondrial-targeted pyrrolo[3,2-d]pyrimidine antifolates for pancreatic cancer. Significance StatementAGF347 is a novel pyrrolo[3,2-d]pyrimidine antifolate that targets serine hydroxymethyltransferase (SHMT) 2 in the mitochondria and SHMT1 and de novo purine biosynthesis in the cytosol. AGF347 accumulation increases with folylpolyglutamate synthetase (FPGS) levels in both the cytosol and mitochondria. Increased mitochondrial FPGS stimulated one-carbon metabolic fluxes in the cytosol and mitochondria and substantially enhanced in vitro inhibition of pancreatic cancer cells by AGF347. Mitochondrial FPGS levels play important roles in determining the anti-tumor efficacies of pyrrolo[3,2-d]pyrimidine antifolates for pancreatic cancer.